This invention relates generally to chemical vapor deposition of semiconducting thin films, and, more specifically, to deposition of films of materials containing tellurium.
Thin, single-crystal films of doped semiconductor materials play important roles in many devices such as electronic components, lasers and detectors for detecting particular types of radiation. The development of these devices depends upon the ability to fabricate thin-film crystals having the required chemical composition, level of dopants, degree of crystalline perfection, surface quality, and thickness uniformity over large areas. The thin films are typically on the order of 10 microns or less in thickness, and are fabricated by deposition on a substrate.
Materials formed from elements in groups II, III, and IV of the periodic table (and their related subgroups such as IIa and IIb), together with elements in the group VI, have been found to have important semiconducting properties that can be applied in a variety of devices. Among such (II, III, IV-VI) materials are those wherein the group VI component is entirely or partly tellurium, these materials being termed (II, III, IV-tellurium) materials. Such materials can be compounds with narrowly defined composition limits or solid solutions with variable composition ranges.
One class of (II, III, IV-tellurium) materials of particular interest is based upon cadmium telluride. Doped cadmium telluride thin films have found particular application in electronic devices. Doped mercury cadmium telluride thin films are nearly ideal for detecting radiation in the near and far infrared ranges. Focal plane arrays for far infrared detectors can be fabricated in such films. The promise of mercury cadmium telluride in this and other applications such as far-infrared detecting diodes and superlattices has been demonstrated, but the ability to fabricate the required thin film crystals with sufficiently large lateral dimensions and uniform thicknesses has yet to be established.
One approach to the fabrication of (II, III, IV-tellurium) materials such as cadmium telluride and mercury cadmium telluride is chemical vapor deposition, wherein reactant source vapors containing the various constituents of the material are reacted at the surface of the selected substrate. Chemical vapor deposition offers the potential for close control of the material properties of the film such as chemistry, dopant levels and crystalline perfection, and of the physical properties of the film such as surface quality and thickness uniformity. With the proper selection of the reactant source vapors and reaction conditions, such a reaction results in deposition of a thin film of the material of interest onto the surface of the substrate, with the undeposited portions of the reactant source vapors being carried away from the substrate and out of the system in a flowing gas stream. The energy to effect the reaction can be supplied by heat or other means such as light. The success of this chemical vapor deposition technique for any particular material depends directly upon selection of the proper reactant sources for the substances to be codeposited.
Focusing on one particular example, in order to deposit cadmium telluride, there must be reactant sources of cadmium and tellurium, while to deposit mercury cadmium telluride, there must further be a reactant source of mercury. Reactant sources for all three of these components are known, and successful chemical vapor deposition of both cadmium telluride and mercury cadmium telluride has been accomplished, at substrate temperatures of about 350 C. and higher.
Chemical vapor deposition of laterally large thin films of (II, III, IV-tellurium) materials such as those based upon cadmium telluride at a substrate temperature of less than 300.degree. C., and preferably about 250.degree. C., would be highly desirable, but has heretofore not been possible by unassisted pyrolysis. When conventional deposition temperatures of 350.degree. C. and higher are used, there can be significant diffusion of atoms within the structure and between adjacent regions of different composition and dopant levels, thereby altering the crystal perfection and reducing intentional internal composition gradients that are engineered into the thin films to produce specific electronic properties, as in the preparation of heterojunctions.
The ability to deposit such thin films at low temperatures is currently limited by the unavailability of reactant sources of tellurium having vapors that decompose at low temperatures. Currently preferred tellurium sources are the organometallic compounds dimethyltelluride and diethyltelluride, neither of which achieves complete thermal decomposition below 450.degree. C. Additionally, these compounds can contain oxygen impurities which can cause polymerization of tellurium if photocatalysis is used to provide non-thermal energy to reduce the deposition temperature. Even if only a partial decomposition is accepted, the lowest deposition temperatures that can be achieved with known reactant sources of tellurium in unassisted pyrolysis is about 350.degree. C.
There have been proposed several approaches to achieving reducing decomposition temperatures for thin films of materials such as cadmium telluride. Light energy can be supplied to assist in decomposition in a process termed photocatalysis, but deposition rates and uniformity of the film being deposited are difficult to control. The films can be grown at low temperatures where decomposition is incomplete, but there may result excess carbon contamination and incorporation of second phases of unreacted metal alkyls in the film. Extremely precise control of the substrate temperature is also required, since the film growth rate is temperature dependent. Slight variations in substrate temperature across a large substrate can result in film thickness variations, which can render the resulting film unusable, as, for example, in a focal plane array detector.
Decomposition of the tellurium reactant source vapor can also be promoted by heating the walls of the chemical vapor deposition reactor, but this can lead to heterogeneous nucleation in the gaseous phase and premature condensation. Yet another approach is to use elemental tellurium as the source of tellurium, but the walls of the gas supply system and the reactor must be heated, leading to the same problems as just discussed.
Thus, while it is possible to prepare thin films of (II, III, IV-tellurium) materials such as cadmium telluride and related materials by chemical vapor deposition techniques, the known approaches all involve deposition at excessively elevated temperatures or the use of modifications that introduce unacceptable side effects. There exists a need for a method of preparing such thin film materials, having controllable chemistry and dopant levels, and a high degree of crystalline perfection, and in a physical form having an acceptably smooth surface and uniform thickness, all of these features being attainable in films of large lateral dimensions for use in devices such as detectors. The method should also be reproducible and economical, with an acceptably high growth rate of the film. The present invention fulfills this need, and further provides related advantages.